This invention relates to the deposition of main group metals, such as aluminum, or of transition metals, such as nickel, either as a substantially pure metal or in combination with boron. The invention is more particularly directed to processes for deposition of metal films and boron-containing metal films which use a metal borane cluster compound as a precursor. The process of this invention can yield metal boride thin films in which the atomic percentage of boron is selected to a desired level.
The deposition of binary transition metal-main group thin-film materials of controlled stoichiometry is currently an area of intense interest. Numerous techniques have been studied for the preparation of these materials including molecular beam epitaxy (MBE), sputtering, and chemical vapor deposition (CVD). The control of the stoichiometry in multicomponent films prepared by chemical vapor deposition techniques has relied on varying the ratio of individual source components in the vapor phase. These source materials typically deposit at significantly different rates on the substrate at any given temperature, often making the formation of a homogeneous film difficult.
Nickel boride thin films have found a number of important applications, beginning with not only traditional uses such as hard cutting tools and inert coatings to protect sensitive devices from severe environmental conditions. NiB films can also be used in fields such as high-energy optical systems and magnetic materials. In addition, the incorporation of boron in nickel films has been shown greatly to enhance the strength and hardness of the alloy. Relatively strain-free nickel boride thin films have been reported from the pyrolytic CVD of gaseous mixtures of nickel tetracarbonyl, Ni(CO).sub.4, diborane, B.sub.2 H.sub.6, and carbon monoxide in an argon carrier. Nickel tetracarbonyl Ni(CO).sub.4 and diborane B.sub.2 H.sub.6 are relatively expensive and extremely toxic and flammable reagents. An alternative to this method which provides stoichiometric control, and yields strain-free films and conformal materials is therefore highly desirable.
The deposition of thin films of main group materials such as GaAs and Al.sub.x Ga.sub.1-x As using metal-organic epitaxial source materials is a subject of high interest. Research in this field has dealt primarily with the development of new epitaxial techniques and the purification of existent organometallic source materials. While these source compounds have proven adequate in many instances, further developments and improvements in epitaxial processes involving new source materials are yet to be developed which exhibit enhanced chemical properties for depositional processes. Thus far, relatively little emphasis has been placed on this vitally important aspect of epitaxial science.
Until now, metal thin film epitaxy has primarily employed deposition techniques using well-known and, typically, commercially available carbon-based source compounds. There are, however, important problems associated with the epitaxial source materials currently in use. These problems include: (1) severe substrate reactivity and bonding problems, (2) high toxicity (and environmental and occupational concerns), (3) difficulties in handling and manipulation, and (4) undesired co-deposition of carbon and other contaminant elements. In many cases, the films that derive from organometallic precursor compounds contain unacceptably high carbon levels, limiting or degrading the semiconducting properties of the film. Aluminum epitaxial depositions have typically been among the most troublesome in this latter respect. Many of the currently employed aluminum precursor compounds are particularly prone to carbon film contamination problems. With the emergence of ternary structures, such as Al.sub.x Ga.sub.1-x As, as vital electronic materials, the development and practical application of new source materials for the deposition of aluminum is therefore an area of major concern to the electronics industry.
Several organoaluminum precursors have been carefully investigated as CVD source materials. The most intensely studied are trimethylaluminum, Al(CH.sub.3).sub.3 (TMA), triisobutylaluminum, Al(i-C.sub.4 H.sub.9).sub.3 (TiBA) and, most recently, the volatile donor-acceptor complexes of alane, AlH.sub.3 (NR.sub.3).sub.2. The deposited films typically exhibit high carbon contamination and have rough surface morphologies and show significant reactor memory effects.